1. Field of the Invention
The present invention relates to a position detection apparatus, an exposure apparatus, and a method of manufacturing a device.
2. Description of the Related Art
In the conventional photolithography process in manufacturing, for example, a semiconductor device and a liquid crystal display device, a circuit pattern formed on a reticle or a mask (to be representatively referred to as a mask hereinafter) is transferred by exposure onto a semiconductor wafer or a glass substrate (to be representatively referred to as a wafer hereinafter) via a projection optical system.
One commonly used exposure apparatus is of the step & repeat type (this type is also sometimes referred to as a stepper) that sequentially transfers by exposure the pattern of a mask to a plurality of exposure regions via a projection optical system. Another commonly used exposure apparatus is of the step & scan type (this type is also sometimes referred to as a scanner or a scanning exposure apparatus) that repeats exposure in a plurality of regions on a substrate by repeating stepping movement and scanning exposure.
In recent years, the patterns of semiconductor devices and other devices are increasingly becoming finer. To attain this, it is necessary to increase the resolution of a projection optical system. To improve the resolution of an exposure apparatus, one method shortens the exposure wavelength, and another method increases the numerical aperture (NA) of a projection optical system.
As for the method of shortening the exposure wavelength, various kinds of light sources have been developed. For example, a shift from the i-line with a wavelength of 365 nm to KrF excimer laser light with an oscillation wavelength around 248 nm is in progress, and an ArF excimer laser with a shorter oscillation wavelength around 193 nm is under development. A fluorine (F2) excimer laser with a further shorter oscillation wavelength around 157 nm is under development as well.
As for the method of increasing the numerical aperture (NA) of a projection optical system, an immersion exposure method is attracting a great deal of attention. In the immersion exposure method, a substrate (e.g., a wafer) is exposed while the space between the substrate and the final surface of a projection optical system, which is conventionally filled with a gas, is filled with a liquid. The immersion method has the advantage of improving the resolution compared to the conventional method even when a light source with the same wavelength as that of the conventional light source is used. Assume, for example, that the liquid supplied to the space between the projection optical system and the wafer is pure water (Refractive Index: 1.33), and the maximum incident angle of a light beam which forms an image on the wafer is the same between the immersion exposure method and the conventional method. In this case, since the NA of the projection optical system increases to 1.33 times, the resolution in the immersion exposure method is 1.33 times as high as that in the conventional method. In this manner, the immersion exposure method can obtain a resolution corresponding to the case of NA≧1, which is impossible for the conventional method. To realize this immersion exposure method, various types of exposure apparatuses have been proposed.
To keep up with an improvement in resolution of projection patterns, it is also necessary to increase the accuracy of relative alignment between a wafer and a mask in an exposure apparatus.
An exposure apparatus is required not only to have a high resolution but also to function as a high-precision position detection apparatus. Under the circumstance, one frequently used wafer alignment scheme is an off-axis autoalignment detection system (to be simply referred to as an “OA detection system” hereinafter). The OA detection system is positioned at a position different from that of a projection optical system and detects the position of an alignment mark on a wafer without using the projection optical system. The wafer is aligned based on the obtained detection result.
One conventional alignment scheme is a method called TTL-AA (Through The Lens AutoAlignment). This method detects an alignment mark on a wafer using a non-exposure light component with an alignment wavelength via a projection optical system. The TTL-AA has the merit of requiring only small amounts of driving of a wafer stage both during alignment measurement and during exposure because a very short distance (so-called baseline) can be set between the optical axis of the projection optical system and that of the TTL-AA. This makes it possible to reduce measurement errors attributed to a fluctuation in the distance between the optical axis of the projection optical system and that of the TTL-AA in response to an environmental change in the vicinity of the wafer stage. In other words, the TTL-AA has the merit of suppressing a fluctuation in baseline.
However, a shift of the exposure light to light with a relatively short wavelength, such as KrF laser light or ArF laser light, leads to limitation of the type of usable glass material, and this makes it difficult to correct the chromatic aberration of the projection optical system for the alignment wavelength. Hence, an OA detection system that is free from the adverse influence of the chromatic aberration of a projection optical system is becoming important.
One alignment scheme described in Japanese Patent Laid-Open No. 2004-279166, which includes a conventional OA detection system, will be explained. FIG. 1 is a schematic view showing the arrangement of a conventional OA detection system.
In the OA detection system, light guided from a light source 1 passes through illumination relay optical systems 2 and 3 and forms an image in an aperture stop formed in a rotary table 4. Specific light having passed through the aperture stop further passes through an illumination optical system 5 and is guided to a polarizing beam splitter 6. S-polarized light reflected by the polarizing beam splitter 6 passes through a relay lens 7 and λ/4 plate 8, is converted into circularly polarized light, and Kohler-illuminates a wafer mark WM, formed on a wafer W, upon passing through an objective lens 9. Reflected light, diffracted light, and scattered light generated by the wafer mark WM travel back through the objective lens 9, λ/4 plate 8, and relay lens 7. The resultant light is then converted into P-polarized light, passes through the polarizing beam splitter 6, and forms an image of the wafer mark WM on a sensor (image sensor) 11 by an imaging optical system 10. The position of the wafer W is detected based on the position of the photoelectrically converted image of the wafer mark WM.
On the other hand, light emitted by a reference plate light source 12 Kohler-illuminates a reference plate 14 by a reference plate illumination optical system 13 so as to generate a uniform light amount distribution on the reference plate 14. A reference mark SM is formed on the reference plate 14, and only light transmitted through the reference mark SM is guided to a half mirror 15. The light source 1 which emits alignment light and the reference plate light source 12 which emits reference light are provided as separate light sources to prohibit emission of reference light when the wafer mark WM is illuminated. Also, emission of alignment light is prohibited when the reference mark SM is illuminated to make it possible to form the wafer mark WM and reference mark SM within the same field of view.
An exposure light scope (not shown) detects the relative position between a mark formed on a wafer stage and a mark formed on a reticle. After that, the so-called baseline is measured by detecting the relative position between the mark on the wafer stage and the reference mark SM by the OA detection system. The reference mark SM serves as a reference of the OA detection system in measuring the baseline. After the baseline is measured, the position of the wafer mark WM is detected with reference to the reference mark SM.
There exists a phenomenon in which the measurement value, that is, the detection position of the alignment mark horizontally varies from the optical axis depending on the Z position in the focus direction as the position in the optical axis direction of the OA detection system, and this variation turns into alignment measurement error components of, for example, the OA detection system. This characteristic in which the detection position horizontally varies from the optical axis will be referred to as a “defocus characteristic” hereinafter.
A defocus characteristic will be explained below with reference to FIGS. 2A and 2B. As shown in FIG. 2A, when the incident angle of the illumination light on the wafer tilts with respect to the wafer, the measurement value of the position of the alignment mark deviates by Δ1 at a defocus of +D [μm] on the plus side or by Δ2 at a defocus of −D [μm] on the minus side. Therefore, the measurement value of the alignment mark depends on the amounts of defocus by:
Plus Side: Δ1/+D Per 1-μm Defocus
Minus Side: Δ2/−D Per 1-μm Defocus
To suppress this defocus characteristic, it is necessary to bring the incident angle of the illumination light on the wafer W close to zero, as shown in FIG. 2B. If the OA detection system measures the alignment mark while exhibiting such a defocus characteristic, a variation in position of the alignment mark in the Z direction turns into that in the measurement direction, and this deteriorates the measurement reproducibility. To combat this situation, it is a conventional practice to prevent generation of any defocus characteristic as much as possible by adjusting the optical axis of the detection light or that of the illumination light, as described in Japanese Patent Laid-Open No. 10-022211.
Japanese Patent Laid-Open No. 10-022211 adjusts the defocus characteristic of a reference mark and detects the position of a mark to be actually aligned, which is assumed to have a defocus characteristic equal to that of the reference mark.
Also, an alignment mark on a wafer is measured by selecting one of a plurality of illumination conditions in order to perform the measurement in conformity with the manufacturing process conditions such as the type and thickness of a resist applied on the wafer. Moreover, an alignment detection system is configured such that the illumination conditions such as the detection wavelength and the NA are variable so as to allow detection with high accuracy for even various kinds of alignment marks corresponding to the conditions involved. More specifically, the wavelength range of the illumination light or the NA of the projection optical system is typically adopted as the illumination condition. In this case, a method of measurement by selecting wavelengths conforming to the conditions involved from a plurality of types of wavelength ranges is employed.
These conventional methods use a plurality of aperture stops and a plurality of light sources to change the illumination condition. A plurality of types of illumination conditions are set by combining these aperture stops and light sources, so it is necessary to reduce measurement errors (defocus characteristic) upon defocusing attributed to the tilt of the optical axis for all these illumination conditions (Japanese Patent Laid-Open Nos. 2003-142375 and 2004-356193).
To meet this requirement, it is necessary to adjust a displacement (decentering) of an aperture stop for an illumination system with respect to an aperture stop for a detection system (corresponding to an aperture stop for an imaging system). One proposed method adjusts the displacement by driving a mechanism which switches between the plurality of aperture stops for the illumination system and these aperture stops for the illumination system in two orthogonal directions to adjust their positions.
Note that the plurality of aperture stops for the illumination system are switched by a driving system. In one example, the driving system has an arrangement in which the patterns of the plurality of aperture stops for the illumination system are formed in a glass disk, and the disk is rotationally driven by a motor, thereby positioning an appropriate one of these aperture stops on the optical axis. The motor used is a pulse motor and a photo-switch is used to detect the origin of the glass disk having the stop patterns formed in it. Because the plurality of aperture stops for the illumination system are switched or adjusted using the driving system, a considerable amount of driving error naturally occurs. For this reason, the defocus characteristic may inevitably remain to some extent in an actual detection system due to the foregoing factors.
There is another problem that the defocus characteristic changes depending on the illumination condition and therefore cannot always be reduced for all wafers. This poses a problem that deterioration in alignment measurement accuracy occurs for certain wafers (especially wafers in different processes) due to the remaining defocus characteristic and a variation in position of the alignment mark in the Z direction, and this, in turn, deteriorates the overlay accuracy.
Japanese Patent Laid-Open No. 2005-026461 proposes another method of adjusting the above-mentioned defocus characteristic. This method observes both an aperture stop for an illumination system and an aperture stop for an imaging system, which are mounted in a position detection apparatus, thereby adjusting decentering of the aperture stop for the illumination system with respect to the aperture stop for the imaging system.
In the first embodiment of Japanese Patent Laid-Open No. 2005-026461, both an aperture stop for an illumination system and an aperture stop for an imaging system are observed at once by switching the aperture stop for the illumination system, from the one which satisfies σ<1 to the one which satisfies σ≧1, thereby adjusting decentering of the aperture stop for the illumination system with respect to the aperture stop for the imaging system. Note that σ is the ratio ((Diameter of Aperture Stop for Illumination System)/(Diameter of Aperture Stop for Imaging System)) of the diameter of the aperture stop for the illumination system to that of the aperture stop for the imaging system, which takes the imaging magnification into consideration at the position of the aperture stop for the imaging system. For this reason, after the aperture stop for the illumination system, which satisfies σ≧1, is adjusted, it is necessary to switch the aperture stop for the illumination system again to the one which satisfies σ<1 for use in mark measurement. It is therefore impossible to compensate for errors attributed to this switching. Furthermore, a diffusing plate is inserted in the optical path to satisfy σ≧1 in the second embodiment of Japanese Patent Laid-Open No. 2005-026461, resulting in complication of the apparatus.